Thin film quantum materials

Abstract:

Magnetic multilayers offer diverse opportunities for the development of ultrafast functional devices through advanced interface and layer engineering. Nevertheless, a method for determining their dynamic properties as a function of depth throughout such stacks have remained elusive. By probing the ferromagnetic resonance (FMR) modes with element-selective soft x-ray resonant reflectivity, we gain access to the magnetization dynamics as a function of depth. Most notably, using reflectometry ferromagnetic resonance (RFMR), we find a phase lag between the coupled ferromagnetic layers in [CoFeB/MgO/Ta]4 multilayers, which is invisible to other techniques. RFMR enables the time- and layer-resolved probing of the complex magnetization dynamics of a wide range of functional magnetic heterostructures with absorption edges in the soft x-ray wavelength regime.

Abstract:

The emergence of magnetic skyrmions, topological spin textures, has aroused tremendous interest in studying the rich physics related to their topology. While skyrmions promise high-density and energy-efficient magnetic memory devices for information technology, the manifestation of their nontrivial topology through single skyrmions and ordered and disordered skyrmion lattices could also give rise to many fascinating physical phenomena, such as chiral magnon and skyrmion glass states. Therefore, generating skyrmions at designated locations on a large scale, while controlling the skyrmion patterns, is the key to advancing topological magnetism. Here, a new, yet general, approach to the “printing” of skyrmions with zero-field stability in arbitrary patterns on a massive scale in exchange-biased magnetic multilayers is presented. By exploiting the fact that the antiferromagnetic order can be reconfigured by local thermal excitations, a focused electron beam with a graphic pattern generator to “print” skyrmions is used, which is referred to as skyrmion lithography. This work provides a route to design arbitrary skyrmion patterns, thereby establishing the foundation for further exploration of topological magnetism.

Abstract:

We report the magneto-optical Kerr effect (MOKE) study of magnetic topological insulator superlattice films with alternating transition-metal and rare-earth doping. We observe an unexpected hump in the MOKE hysteresis loops upon magnetization reversal at low temperatures, reminiscent of the topological Hall effect(THE) reported in transport measurements. The THE is commonly associated with the existence of magnetic skyrmions, i.e., chiral spin textures originating from topological defects in real space. Here, the observation of the effect is tied to ferromagnetic ordering in the rare-earth-doped layers of the superlattice. Our study may provide a new approach for the non-invasive optical investigation of skyrmions in magnetic films, complementary to electrical transport measurements, where the topological Hall signal is often the only hint of non-trivial magnetization patterns.